Is an air oil separator a good mod for these cars?
It isn't detrimental, but I wouldn't worry about it (IAG runs ~$400) on a simple S2 car.
Cobb offers some maps for basic modifications, but they are not ideal. You're always going to be better off having a calibrator get their hands on your vehicle and dial in ECU mapping specific to your vehicle. There are some gains to be had from custom calibration on the S2+SF hardware, but don't expect miracles.
Once you've done S2+SF modifications, if you still want a little bit more, consider the following (this will require custom calibration, so stockpile parts until you've bought everything and get one tune):
1) ID1050X injectors / Fuel Pump. At S2+SF levels, you're maxing out your fueling system, to the point that you're not able to take advantage of the additional air from the Cobb SF intake. While the injectors are "overkill" for what you'd need on pump gas, it would give you the option of running E85. If you plan on doing E85 at this time, you may want to look into Cobb's Flex Fuel kit, which will allow your ECU to interpolate between your 91 / E85 tables based on the ethanol content that you have, without having to reflash the ECU. Innovative Tuning did a good before/after test
with intake and injectors that shows what kinds of gains you can get when you have adequate fueling. It was with a different intake and injectors than you have and I'm recommending, but the power gains you'd be looking at are similar.
2) EL manifold. While some people love the "Boxer Rumble", there are serious gains to be had with a good EL manifold. The sound will change (think Porsche), but it's absolutely worth it; I made the switch last year and wish I'd done it sooner. TopSpeed did a good before/after test
with an Invidia manifold. Their test also includes an uppipe, but that install is a PITA, and the majority of the gains are from the manifold, not the uppipe. Unless you want to spend $500-600 for an external wastegate uppipe, don't bother replacing the OE version unless the flex section gives out. EWG setups offer their own benefits
, but for the purpose of this, I'm excluding it from the list to keep costs down.
3) Aftermarket boost control. The factory boost control solenoid is sufficient for stock turbo setups, but talk to the calibrator to see if they would prefer an aftermarket MBC or 3-port BCS. You're looking at an additional ~$100 (or less, if you go with the more "DIY" GM option
), and if they want you to have it, just spend the money, because you're already going to be in for ~$2K in parts / tuning at this point. A lot of people will buy a P&P unit like Grimmspeed simply for the ease of install, but in general, the typical 3-port BCS will all perform pretty similar. If you decide to add EWG to #2, absolutely go with aftermarket boost control.
When it comes to handling, there are three steps you should take:
1) Tires. You mentioned having a set of MPSS (solid tire), so you're good there.
2) Alignment. Factory alignment isn't designed with performance as the #1 criteria. Adding in some negative camber up front can help take out some of the understeer that everyone talks about.
3) Aftermarket parts. The parts list here is vast, based on what you're planning to do with the car. Certain suspension modifications require alignment (e.g., struts/springs, certain bushings, etc.), so may be worth compiling those parts and installing all at once vs doing so incrementally.
Other people have mentioned driveline modifications. I am a big advocate for these items, as they will make every time you drive your car feel better. If you're just putting around town, you will get the better feel out of these items, where you won't necessarily see the power gains from the "go fast" parts. I outlined a bunch of them here: http://www.clubwrx.net/forums/transmission-awd/134325326-driveline-mounts-bushings-faq.html
. A lot of the install write-up links are dead, but the descriptions still apply. The nice thing about driveline modifications is that they are relatively inexpensive (compared to other items like exhaust, injectors, etc.), so bang/buck is quite high.